1. Field of the Invention
The present invention relates to a fluorescent image obtaining apparatus for obtaining an image formed of the fluorescent light emitted from a measurement area upon the irradiation thereof by an excitation light, and which represents the data relating to a living-tissue subject.
2. Description of the Related Art
Fluorescent-light detection apparatuses have been proposed that make use of the fact that the intensity of the fluorescent light emitted from a normal tissue differs from the intensity of the fluorescent light emitted from a diseased tissue when a target subject is irradiated by an excitation light having a wavelength within the wavelength range of the intrinsic fluorophores of the target subject, wherein, by detecting the fluorescent light emitted from a target subject upon irradiation thereof by a excitation light having a wavelength within the wavelength range of the intrinsic fluorophores of the target subject, the location and range of penetration of a diseased tissue is discerned.
Normally, when a target subject is irradiated by a excitation light, because a high-intensity fluorescent light is emitted from a normal tissue, as shown by the solid line in FIG. 6, and a weak-intensity fluorescent light is emitted from a diseased tissue, as shown by the broken line in FIG. 6, by measuring the intensity of the fluorescent light emitted from aforementioned target subject, it can be determined whether the target subject is in a normal or a diseased state.
Further, methods have been proposed wherein the fluorescent light emitted upon the irradiation of a target subject by the excitation light is obtained as an image by a obtaining element or the like, and displayed as a fluorescent image corresponding to the intensity of the fluorescent-light. Further, according to the technology described above, because there is unevenness on the surface of a target subject, the intensity of the excitation light irradiating the target subject is not of a uniform intensity. Further, although the intensity of the fluorescent-light emitted from the target subject is substantially proportional to the intensity of the excitation light, the intensity of aforementioned excitation light becomes weaker in inverse proportion to the square of the distance between the excitation light and the target subject. Therefore, there are cases in which the fluorescent-light received from a diseased tissue located at a position closer to the excitation light source than a normal tissue is of a higher intensity than the fluorescent-light received from aforementioned normal tissue, and the state of the tissue of the target subject under examination cannot be accurately discerned based solely on the data relating to the intensity of the fluorescent-light received from the target subject upon the irradiation thereof with a excitation light. In order to remedy the problems described above, methods such as dividing two types of fluorescent-light intensities obtained of different wavelength ranges (a narrow-band near the 480 nm wavelength, and a wide band near the 430–730 nm wavelength range) to obtain the ratio therebetween, and displaying a computed-image based on the factor obtained thereby have been proposed. That is to say: an image display method of displaying an image based on the difference in the form of the fluorescent-light spectra reflecting the tissue-state of a target subject; a method of displaying a fluorescent image comprising detecting the intensity of the reflected-light reflected from a target subject upon the irradiation thereof with a reference-light composed of light in the near-infrared spectrum, which shows uniform absorption characteristics for awide variety of target subjects; obtaining the ratio between the intensity of the reference-light and the intensity of the fluorescent-light by division; and displaying a computed-image based on the factor obtained thereby; that is, methods such as obtaining a value reflecting the yield of the fluorescent-light and displaying an image; have been proposed. Further: a method of assigning color data to the factor obtained by dividing the intensities of the fluorescent light of different wavelength bands or to the factor obtained by dividing the intensity of the fluorescent light by the intensity of the reflected-light reflected from the target subject upon the irradiation thereof by the reference-light, and displaying an image representing the tissue-state of the target subject based on the difference in the colors thereof; a method of displaying a composite-image, which is an image imparting a sense of the surface roughness and which also reflects the tissue form of the target subject, formed by combining a color image based on said difference in color representing the tissue-state and a brightness image obtained by assigning brightness data to the intensity of the reflected-light reflected from the target subject upon the irradiation thereof by the reference-light; etc., have been proposed.
However, according to the technology described above, because the intensity of the fluorescent light emitted from the target subject is extremely weak, the S/N ratio of an image based on this fluorescent light is extremely poor. Accordingly, to date, a method for controlling the gain of an amplifying obtaining element for amplifying the intensity of the fluorescent light has been proposed as a method of improving the S/N ratio of a fluorescent image obtained based thereupon; however, according to this method, the intensity of the noise is amplified together with the intensity of the fluorescent light. Further, a method of changing the binning size of the obtaining element to an active binning size corresponding to detected fluorescent light has also been proposed, however, according to this method, although the intensity of the fluorescent light is amplified, because the dark noise of the peripheral pixels is also amplified when the pixels are coupled, the resulting S/N ratio is insufficient. Still further, there is a drawback in that the drive circuit of the obtaining element for changing the binning size is complicated.